1. Field of the Invention
The present invention relates to an image reader such as a digital copying machine, a facsimile and a scanner and its image reading method.
2. Description of the Prior Art
There have been provided various types of image readers such as a digital copying machine, a facsimile and a scanner which read an original to be conveyed by a conveying device in a predetermined reading position. In these types of image readers, dust particles or the like may adhere to a reading unit. In such cases, when an image is read, the dust particles are read by the reading unit, and therefore, streaks, which are not in the original image but extend in the slow-scanning direction, appear on an output image or a transmitted image (hereinafter, simply referred to as “output image”) to be obtained from the image reader.
As a way of solving this problem, there have been proposed a method of giving processing for preventing dust particles or the like from adhering to the surface of contact glass on the reading unit, a method of setting the position of the reading unit to a place where dust particles hardly adhere and the like. These methods, however, cannot overcome a drawback which occurs when dust particles have adhered to the reading unit, that is, a drawback in which streaks due to adhesion of dust particles appear on the output image.
A technique which prevents, when dust particles adhere to the reading unit, the dust particles from affecting an output image has been proposed in, for example, Japanese Published Unexamined Patent Application No. 9-139844.
The outline of an image reader disclosed in the Japanese Published Unexamined Patent Application No. 9-139844 is as follows: First, in the present image reader, an original under conveyance is read in two reading positions slightly apart from each other along a conveying direction of the original. In this in respect, for the sake of convenience in the following description, the first reading position, which the original under conveyance passes through, is called an “upstream-side reading position” and the second reading position, which it passes through, is called a “downstream-side reading positions”.
When an image is in this manner read from the original in the two places: upstream-side reading position and downstream-side reading position, in the upstream-side reading position, image data on each fast-scanning line which has been arranged in a slow-scanning direction as shown in, for example, the following:                Pk, Pk+1, Pk+2, Pk+3 . . .are sequentially obtained.        
In contrast, in the downstream-side reading position, image data, which is, for example, d lines behind in phase than this image data, is obtained as follows:                Pk+d, Pk+d+1, Pk+d+2, Pk+d+3 . . . In this respect, a suffix in the image data Pk or the like in this example is the number of the fast-scanning line.        
If the assumption is made that dust particles adhered only to a position corresponding to the downstream-side reading position on the contact glass, image data faithful to the original image could be obtained from the upstream-side reading position, while image data affected by dust particles would be obtained from the downstream-side reading position and a difference would occur between both image data.
Thus, in the present image reader, with respect to the image data in the upstream-side reading position, image data having the same phase as the image data in the downstream-side reading position is generated by imparting a delay corresponding to the above-described phase delay d, and this image data is compared with the image data in the downstream-side reading position. If there is a difference between the two, it will be determined that dust particles have adhered to the downstream-side reading position.
Also, in this case, it can be said that, of the image data in the downstream-side reading position, a portion different from the image data in the upstream-side reading position is image data in a portion affected by dust particles. Thus, in the present image reader, by replacing the image data in the portion affected by the dust particles with fixed mask data, streaks to be appeared on the output image are removed.
In the configuration disclosed in the above-described Japanese Published Unexamined Patent Application No. 9-139844, image data pieces in the downstream-side reading position and the upstream-side reading position are binarized respectively, and whether or not dust particles have adhered is determined depending on whether or not these binarization data pieces are different. For this reason, only when either of each image data piece in the downstream-side reading position or the upstream-side reading position is higher than a binarization threshold and the other is lower than the threshold, it is determined that the dust particles have adhered. When both image data pieces exceed the threshold or when they do not exceed the threshold, there is the problem that it is not determined that the dust particles have adhered even if the dust particles have adhered.
As a method which solves this problem and precisely determines the adhesion of dust particles, there can be conceived a method which compares each image data (multilevel data) in the downstream-side reading position and the upstream-side reading position as it is, and determines that dust particles have adhered if a difference between the two exceeds a predetermined threshold. However, there may take place a steady offset between each image data piece in the downstream-side reading position and the upstream-side reading position resulting from the configuration or the like of the image reader. In a situation in which such a steady offset takes place, there is the problem that even if the adhesion of dust particles is determined on the basis of a difference between each image data piece in each reading position, the determination may become incorrect. With reference to FIG. 1, a detailed description of this problem will be made.
FIG. 1 is a view showing the configuration of an original conveying system in the image reader and an optical system for reading an image. In FIG. 1, an original 2 is carried to a conveying roller 4 one sheet at a time by a drawing-in roller 3. The conveying roller 4 conveys the original 2 to a contact glass 5 by changing the original conveying direction. The original 2 is pressed against the contact glass 5 by a back platen 7, and is finally exhausted from the conveying device by an exhaust roller B. The above-described upstream-side reading position and downstream-side reading position are located above the contact glass 5. A reference numeral A in FIG. 1 designates an original image in the upstream-side reading position, and a reference numeral B, an original image in the downstream-side reading position. These original images A and B have their optical paths changed by a first mirror 9, a second mirror 10 and a third mirror 11 respectively, are reduced by a lens 12, and reach CCD 1.
In the configuration shown in FIG. 1, the original 2 is conveyed obliquely downward to the contact glass 5 by the conveying roller 4. At the time of the original reading, an optical path of an original image A in the upstream-side reading position and an optical path of an original image B in the downstream-side reading position are perpendicular to the contact glass 5, but the upstream-side reading position and the downstream-side reading position are provided within a section in which the original 2 is conveyed obliquely with respect to the contact glass 5. Therefore, an optical path length between the upstream-side reading position and CCD 1 becomes longer than that between the downstream-side reading position and CCD 1. For this reason, even if no dust particles adhere to the contact glass 5, a density value of a read image A in the upstream-side reading position becomes higher than a density value of a read image B in the downstream-side reading position as exemplified in FIG. 12A, and an offset ΔD having a predetermined size takes place between image data corresponding to each reading position.
When a difference obtained by deducting the density value of the read image A in the upstream-side reading position from the density value of the read image B in the downstream-side reading position is compared with the threshold to determine adhesion of dust particles, it becomes difficult to determine the adhesion of dust particles because if dust particles adhere to the contact glass in the downstream-side reading position, a difference between density values of each read image is reduced under the influence of the offset ΔD. FIG. 12B shows its example. If the assumption is made that there is no offset ΔD described above between each reading position, there would take place a comparatively large difference ΔD1 between density values of read images in each reading position if dust particles adhere to the contact glass in the downstream-side reading position. In a configuration shown in FIG. 2, however, since there takes place the offset ΔD between each reading position, the difference between density values of read images in each reading position decreases to ΔD2 from ΔD1, and it becomes difficult to determine the adhesion of dust particles.
In the configuration disclosed in the Japanese Published Unexamined Patent Application No. 9-139844, by replacing image data affected by dust particles with fixed mask data, the streaks on an output image have been removed, but this method has a problem that the output image is deteriorated because a density difference takes place between an image corresponded to the mask data and images all around the image.
As a remedy, there can be conceived a method in which, in a case where dust particles have been found to adhere, image data of the read image, in which, of the read images A and B, it has been found that no dust particles adhere, is selected to thereby obtain image data from which the dust particles have been removed. When, however, this method is executed under such circumstance that the offset ΔD takes place, the image data affected by dust particles is to be replaced with image data having different density therefrom, and the output image will be deteriorated. For example, it is assumed in the configuration shown in FIG. 1 that when it is found that no dust particles have adhered, image data of the read image B in the downstream-side reading position is selected, while when it is found that dust particles have adhered to the downstream-side reading position, image data of the read image A in the upstream-side reading position is selected. In this case, since the image data of the read image A has been selected in place of the image data of the read image B, it is possible to erase clear streaks caused by the dust particles, but thinly black streaks remain in a portion in which the image data has been replaced because a density value for the read image A is higher than that for the read image B.